The present invention relates to the field of cinema sound systems and more particularly it relates to an improved full-frequency-range loudspeaker array in a modular system for providing defined full audience coverage in a theater where the system is deployed behind a conventional perforated cinema screen.
The requirements for a cinema loudspeaker system can be stated simply: to provide uniform sound coverage as perceived at practically all seating locations in the theater with regard to both loudness and flatness of full frequency response, while causing the perceived sound source to coincide with the images projected on the screen, with sufficient overall efficiency to keep the total power requirements within practical limits.
Fulfilling these requirements is far from simple and requires special treatment for various portions of the total audio frequency spectrum and for various regions of the theater auditorium.
A principal design challenge in cinema sound, with the burden falling largely in the mid-frequency range, is the unusual degree of beamwidth confinement and selective control required in efforts to configure and deploy an array of loudspeakers that will satisfy the defined audience coverage requirements from the front to the back row and fully to the sides of the auditorium. Without special selective directivity, the audience coverage would be uneven, and much of the available acoustic energy would become lost, escaping to regions other than the seating area.
For defined coverage, cinema sound systems are required to provide controlled sound directivity, typically measured in a hemispherical free space to approximate the geometric conditions of anticipated final location in the front wall of a theater; the beamwidths at xe2x88x926 dB coverage are typically required to be about 90 to 100 degrees horizontal by 40 to 50 degrees vertical. This beamwidth is a function of the loudspeaker system design at all frequencies down to about 250 Hz; below this, sound inherently becomes increasingly non-directional as the frequency decreases.
The system must meet the requirement of providing spatial accuracy, i.e. identifying the source of sound with differently located images on the screen. Typically this can be addressed satisfactorily by a three channel system having left, center and right vertical arrays or stacks behind the screen, however to preserve xe2x80x9cstereo sound stage imagingxe2x80x9d each of these stacks must be designed for defined coverage of the full theater area. Typically the three stacks are made physically identical, but they may be equalized individually for optimizing coverage and frequency response.
Since a solid screen generally require loudspeakers to be located above or below the screen, the majority of motion picture exhibitors utilize a perforated vinyl screen in order to preserve accuracy of sound sourcing by locating the loudspeaker system behind the screen. However, due to the small diameter holes, the low ratio of open area introduces frequency-dependent anomalies such as attenuation, reflections and beam spreading which degrade the audience coverage.
Frequency-dependent attenuation can be dealt with by equalization, however there are usually associated spatial anomalies that must be addressed as well.
In addition to auditorium acoustics, the region behind the screen and even the space between the screen and the speakers must be considered with regard to harmful reflections.
Beam spreading due to the screen perforations increases with frequency. FIG. 1A is a graph showing target directivity index for a cinema loudspeaker of known art, with no screen present, as being constant at 10 dB, which represents a gain at the strongest direction, typically on the major axis, relative to a omnidirectional point source of the same power. The corresponding beamwidth, shown in the curve of FIG. 1B, is seen to be constant at 100 degrees: cinema loudspeaker systems are typically designed for 90 to 100 degrees horizontal beamwidth. These target parameters have been used conventionally for design and evaluation of cinema loudspeakers with no screen present. However when deployed behind the cinema screen, the directivity index tends to decrease with increasing frequency as shown in FIG. 2A, which shows it reducing to 5 dB above 10 kHz; the corresponding beamwidth, shown in FIG. 2B, spreads to nearly double, increasing from 100 degrees to about 180 degrees which is practically omni-directional in the case of the movie theater since the sound source, i.e. the loudspeaker, is in effect mounted in one wall and thus working into a hemispherical field region.
FIGS. 3A-C are polar graphs showing horizontal directivity as provided by a conventional high-frequency module of known art measured at 2, 4 and 8 kHz respectively, with a cinema screen spaced away 2xe2x80x3, 8xe2x80x3, and completely removed.
FIGS. 3D-F are polar graphs showing the vertical directivity corresponding to FIGS. 3A-C.
This screen beam-spreading effect, increasing with frequency as seen here and in related FIG. 2B, wastes high-frequency audio power emanating in unwanted directions and generally degrades the high-frequency coverage of the system. It remains a problem in the attainment of required beamwidth at high-frequency for defined coverage of cinema sound systems: a problem that has not been adequately addressed in known art. Heretofore, transducer driver units, waveguides and/or stacks thereof for behind-the-screen cinema deployment have not been commercially available with capabilities to fully meet increasingly demanding requirements for defined coverage with full frequency high fidelity and space constraints: more specifically, with compensation for perforated screen spreading in the high-frequency range that increases with frequency and/or with sufficient directivity for defined coverage control in the lower mid-frequency range and/or in a sufficiently compact size for installations where there is only limited space available behind the screen, which can be as little as 18 inches.
U.S. Pat. No. 4,569,076 to Holman for a MOTION PICTURE THEATER LOUDSPEAKER SYSTEM discloses such a system wherein the loudspeaker elements are made to be integral with an acoustical boundary wall constructed behind the screen in order to optimize the characteristics of vented bass woofers.
U.S. Pat. No. 4,580,655 to Keele Jr. discloses a DEFINED COVERAGE LOUDSPEAKER HORN wherein opposed sidewalls are constructed to direct portions of a sound beam toward a target over different preselected incident angles.
U.S. Pat. No. 5,233,664 to Yanagawa et al for a SPEAKER SYSTEM AND METHOD OF CONTROLLING DIRECTIVITY THEREOF discloses the use several different digital filters connected between a common input terminal and several speaker units arranged linearly, in a matrix or in honeycomb form.
U.S. Pat. No. 5,004,067 to Patronis for a CINEMA SOUND SYSTEM FOR UNPERFORATED SCREENS utilizes an exponential middle frequency horn, crossed-over at 150 and 600 Hz, physically combined with a constant directivity high-frequency horn, for mounting three such units above the screen while locating three direct radiator bass units at the floor position beneath the screen.
U.S. Pat. No. 5,020,630 to Gunness for a LOUDSPEAKER AND HORN THEREFOR discloses a high-frequency loudspeaker for projecting sound over a listening area having a driver and a horn in which the horn has a coupling portion communicating with an outwardly flaring portion, the horn forming an elongated slot at the interface, the slot being narrower at one end and flaring outwardly to the other end. The driver frequency range is 500-20,000 Hz, and directivity is shown at 2,000 Hz. This patent teaches a high central loudspeaker location, downwardly inclined at the front end of an auditorium; however it fails to address the particular requirements of theaters or deployment behind a cinema screen.
Speaker arrays, including electronically and/or acoustically filtered arrays have been used in known art for low-frequency pattern control, but have relatively low efficiency, limited bandwidth capability and/or excessive physical size.
It is a primary object of the present invention to provide a loudspeaker system for deployment behind a perforated cinema screen, to accomplish defined uniform sound coverage with regard to loudness and full frequency range as perceived in virtually all listening regions of a theater auditorium.
It is a further object to provide a full-frequency range loudspeaker array system having a shallow profile less than 18 inches in depth.
It is a further object to provide a modular loudspeaker system that satisfies the defined coverage requirements for a theater installation utilizing a horizontal array of several physically identical modular vertical stacks, typically three, each stack formed from separate waveguide acoustically-loaded modules, each of which is dedicated to a different portion of the frequency spectrum, and which can be manufactured, tested, marketed and/or deployed independently.
The abovementioned objects have been accomplished in the present invention in a system of modular stacks that can be deployed in multiples, typically a row of three, behind a cinema screen. Each stack constitutes a three-way vertical line array having a high-frequency module stacked on top of a multi-driver midrange module which in turn is stacked on top of a dual-driver low-frequency module. The crossover frequencies are 250 Hz and 1.2 kHz.
The high-frequency module utilizes a compression driver coupled to a horn waveguide with a special orientation, vertical asymmetry and three-dimensional waveguide shaping to provide controlled directivity that increases with frequency, to compensate for cinema screen spreading and to optimize defined coverage uniformity.
The midrange frequency module is an integrated multi-band waveguide assembly configured to provide a vertical array of four contiguous specially-shaped waveguide regions each driven by a cone type transducer driver. The required defined coverage is accomplished through a combination of special shaping of the waveguide directing surfaces with vertical asymmetry to provide controlled directivity vertically and horizontally, and frequency-selective filtering in a passive network that accomplishes the required overall coverage by splitting the drive power into two paths with different special transfer functions allocated to the lower two transducers as a low-frequency portion and the to the upper two transducers as a high-frequency portion of the midrange assembly. The four drivers are separated by partitions shaped with strategic spacing dimensions, each driver working into an individual waveguide throat portion, and each directed at an inclined angle downwardly from horizontal, to optimize defined coverage uniformity. The throat portions combine smoothly into a common flared mouth portion which extends to the substantially rectangular shape of the front outline of the midrange module.
The low-frequency module is a vented bass enclosure deploying a vertical stack of two 15xe2x80x3 cone type transducers with response extending down to 30 Hz at xe2x88x926 dB.
The resulting cinema loudspeaker system provides high efficiency, high sound level capability and well-controlled coverage, compensated for screen spreading at high-frequency, and maintained substantially constant for beamwidth over the high and midrange frequency range (16 kHz to 250 Hz) in both vertical and horizontal directions. Beamwidth as well as amplitude response are matched at the 250 Hz and 1.2 kHz crossover frequencies for seamless acoustic integration of the three modules. The combination of a waveguide designed for use with a filtered line array with a well-designed filtered line provides significantly better performance than current designs.